Rotary transfer machines are best known for high volume, mass production where multiple machining operations are necessary to complete a part or workpiece. An exemplary use of rotary transfer machines is to machine close tolerances into die cast workpieces. Rotary transfer machines typically rotate and index workpieces from station to station via a rotary index table mounted in the center of the stations. A drive control, typically a geneva drive mechanism or a two face gear coupling, performs intermittent indexing and rotating of the index table to cycle the workpieces sequentially through the stations. Various operations can be performed at each machining station as desired, including but not limited to milling, drilling, cross drilling, boring, internal and external recessing, threading, tapping, broaching and other machining operations.
The rotary index table typically has mounted thereon several clamping pallets that hold workpieces in position. The machine tools may drive a spindle radially inward into the workpiece towards the center of the table, or may work vertically above the workpiece, or may work at an angle therebetween. The clamping pallets may either be fixed relative to the rotary index table or capable of being rotated, tilted or otherwise moved relative to the rotary index table by mounting the pallets on satellite tables that are rotatable relative to the index table. By rotating the satellite table, more sides of the workpiece are exposed to allow for machining of more sides of the workpiece. It is much less desirable to move or rotate the workpiece relative to the index table once it is locked into position for a variety of reasons. One reason is that doing so decreases the tolerances between different machining operations performed at different stations because there are two different axes of rotation which allows for slop between rotational axes. Another reason is that cycle time is increased which results in an inefficiency reducing the production rate of the rotary transfer machine. Yet another reason is the high cost associated with providing the satellite tables and appropriate drive and positioning means for selectively positioning each of the satellite tables.
Utilizing such means as satellite tables on the rotary index table or other such carriage, prior rotary transfer machines have achieved a maximum of five sides of machining. However, it may be necessary to machine the sixth side of the workpiece which requires unclamping and reclamping the workpiece into another machine tool for machining. Not only does this further decrease the precision between machined holes, surfaces, grooves and the like, but it is also labor intensive as more workers are needed for the multiple unloading and loading operations and for operating separate machines. This is costly and time consuming. This also reduces the production rate and disadvantageous from both a fixed and variable cost perspective as additional space, clamping assemblies and machine tooling is necessary for machining the sixth side.
The drive controls of prior rotary transfer machines have also been less than satisfactory. Two face gear couplings require raising and lowering the table each time the table is rotated and indexed to the next station. Two face gear couplings therefore undesirably require a complex range movements in multiple directions which is also time consuming and results in an increase in the time necessary for the index cycle and therefore a decrease in production rates. Geneva drive mechanisms typically operate at fixed speed which undesirably causes low transfer speeds of workpieces from station to station which results in a relatively long indexing cycle. This limits the rate at which workpieces can be machined by the rotary transfer machine. Geneva drive mechanisms have also been known to have problems achieving precise positioning of the index table necessary for machining close tolerances into the work pieces. One such problem is that Geneva drive mechanisms use a stop pin for locating the index table. Stop pins are subject to wear and therefore cause misalignments in the index table over time.